Cooling tray for cast blocks

ABSTRACT

A method is described for indexing a polyform block in which a metal field shaping block is cast to both the cutting tray of a hot wire cutting system and the treatment tray of a radiation therapy treatment machine. The cast metal field shaping block is automatically correctly oriented for treatment. A cooling tray for cooling the casting is also described.

This is a continuation of application Ser. No. 658,434 filed Oct. 5,1984 now abandoned.

BACKGROUND OF THE INVENTION

It is conventional practice in radiation therapy to cast beam-formingfield shaping blocks specifically individualized to each patient. Theeutectic alloy field shaping blocks are usually cast in a polyfoam blocksuch as polystyrene which has been especially cut to make the mold. Onemethod of cutting the field shaping block molds is to place the patientx-ray a set distance from a pivot which simulates the radiation sourceand to place a block of rigid polyfoam a set distance between the x-rayand the pivot and to cut the polyfoam with a hot wire cutter whichfollows either a pivoted rigid rod or a long wire cable as the operatortraces the outline of the designated treatment areas on the x-ray.

The voids thus created in the polyfoam are then filled with a molteneutectic alloy which cools to become one or more field shaping blocks.The blocks are then placed on a treatment tray in the beam of theradiation therapy machine, such as an accelerator or cobalt unit. Thefield shaping blocks are manipulated into position on the treatment trayto shield the areas of the patient which are to be protected fromradiation.

While this current practice of defining the radiation beam is certainlybetter than the older practice of simply arranging rectangular leadblocks on the treatment tray, the whole process leaves much to bedesired, and it leads to many cumulative inaccuracies because theprocess described above cannot orient field shaping blocks withsufficient accuracy for use with modern high-powered, well-collimatedtreatment machines. Even slight errors in orientation can seriouslyundermine the physician's intent by allowing some areas to go untreatedwhile others are seriously over-exposed. Furthermore, in casting theblocks, conventional cooling trays do not provide uniform pressure inholding the polyfoam block, and they permit spillage to occur.

Accordingly, it is an object of this invention to provide an improvedsystem of indexing and orienting accurately formed cast field shapingblocks in radiotherapy treatment apparatus. It is another object toprovide a cooling tray for cast field shaping blocks.

DESCRIPTION OF THE INVENTION

The invention provides an indexing system which automatically ensurescorrect placement of the field shaping eutectic alloy block in theradiotherapy treatment machine in a position exactly analogous to thatof the cavity in which the field shaping block was cut from thestyrofoam block in the hot wire cutting machine using, advantageously,the hot wire cutting system described in my copending patentapplication, Ser. No. 658,435, filed Oct. 5, 1984, now U.S. Pat. No.4,601,224.

Unless the cast alloy shielding blocks are correctly situated upon thetreatment tray, all the accuracy in cutting has been wasted. It istherefore paramount to orient the shielding blocks in the correctposition on the radiation treatment tray. Accordingly, it is a purposeof this invention to index the polyfoam block to the cutting tray usedin the cutting system and to the treatment tray used in the treatmentmachine in such manner that the shielding blocks cast within thepolyfoam block will automatically be correctly oriented on the treatmenttray. The shielding blocks can be fastened to the tray in correctposition before the polyfoam is removed.

A method is described of indexing to a treatment tray used in aradiation therapy treatment machine having a radiation source and apatient positioned for treatment in line with the radiation source, apolyfoam block having a cavity cut in it by cutting apparatus having aphantom radiation source and a patient x-ray film positioned in linewith the phantom source, the cavity being filled with a cast metal fieldshaping block. The polyfoam block containing the cast metal fieldshaping block is indexed to the treatment tray of the radiation therapytreatment machine with respect to the radiation source and positionedpatient identically to the indexation of the polyfoam block to a blocksupport or cutting tray in the cutting apparatus with respect to thephantom radiation source and patient x-ray film. The polyfoam block maybe indexed to the treatment tray by an arrangement of fasteners, thearrangement of fasteners being identical to an arrangement of fastenersfastening the polyfoam block to the block support or cutting tray in thecutting apparatus.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a partially exploded supporting system for indexing thepolyfoam block to the cutting machine tray.

FIG. 2 shows a tray suitable for pouring and cooling a cast metalshielding block.

FIG. 3 shows an exploded view of an indexing system for securing thepolyfoam block on the treatment tray.

FIG. 4 shows a secured metal block cast in a secured polyfoam block onthe treatment tray.

FIG. 5 shows a partially cut away cast metal block secured to thetreatment tray.

A polyfoam block such as polystyrene foam is used for cutting thecavities in which the shielding blocks used in radiation therapy arecast because it has a high degree of dimensional stability. An indexingmethod which capitalizes upon this stability and rigidity is desirable.Accordingly, a convenient method to index the polyfoam block to thecutting machine tray and the treatment machine tray is to deform anunused area of the polyfoam block in such manner that the polyfoam blockwill index to the cutter tray and to the treatment tray by interlock tothe deformation.

Any of several practical and simple arrangements are suitable for this"lock and key" index system. For example, both the cutter tray and thetreatment tray may have a series of raised pins which indent thepolyfoam, thus assuring precise location. Alternatively, the arrangementof FIGS. 1-5, in which like parts are similarly numbered, offers theadvantage of increased mechanical stability because it holds the foamblock firmly in place.

In FIG. 1 there is shown vertical column 5 supporting a cutter tray 4which holds a block of polyfoam 2 shown with a cut pattern forming acavity 3 and cutter wire 1. The polyfoam block 2 is held in the traywith two or more thumb screws 7 which pass into the polyfoam block 2through holes 6 in the cutter tray 4.

The dimensional stability of the rigid polyfoam makes it ideal formolding and indexing the shielding blocks. However, because the polyfoamis lightweight, inelastic, and easily crushed or deformed, it has longpresented great difficulties in the casting process itself.

While the polyfoam is light in weight, the eutectic alloy is very heavy,approximating the weight of lead. It is consequently necessary to holdthe polyfoam mold firmly against a cooling surface while the moltenalloy is poured into the cavity in the polyfoam or the alloy willdisplace the polyfoam, spill out, and spoil the casting. While variousclamps and weights have previously been devised to immobilize thepolyfoam, none have proven truly satisfactory, because either they crushthe polyfoam, or they do not maintain the polyfoam in uniform, intimatecontact with the cooling surface. The conventional method is to tapeshut the "closed" end of the mold cavity. This method not only wastestime, but it also greatly retards cooling.

In order to eliminate these problems, it is essential to provide uniformpressure on the polyfoam so that the casting is not distorted, yet thepressure must be sufficient to intimately immobilize the polyfoam blockagainst the cooling surface. Furthermore, the cooling surface mustconduct heat rapidly and efficiently so that the eutectic alloysolidifies rapidly and forms its own seal as the remainder of the alloyis poured.

A cooling tray of this invention for cooling a field shaping block castin a cavity in a polyfoam block comprises a generally flat heat-sinkbase means for supporting the polyfoam block on an upper surfacethereof, a spaced pair of clamp support means affixed on the coolingtray for embracing respectively opposite sides of the polyfoam blockdisposed in the space therebetween, a pair of angle clamps eachcomprising first and second flanges disposed at right angles to oneanother, hinges pivotly supporting the first flanges of the angle clampson the clamp support means with the second flanges thereof disposed overthe space between the clamp support means, and spring means for biasingthe angle clamps to pivot about the hinges and normally biasing theangle clamps to pivot so that the second flanges thereof are forceddownwardly toward the space between the clamp support means, whereby toforce the second flanges downwardly against the polyfoam block supportedon the tray between the clamp support means.

FIG. 2 shows a cooling tray 12 which holds the polyfoam block 2 whilethe metal casting is poured and cooled. The cooling tray 12 has aheat-sink cooling surface such as brushed or etched aluminum, and isshown with a block of polyfoam 2 immobilized upon it by means of two ormore angle clamps 13. Angle clamps 13 seize polyfoam block 2 along itsedges and distribute clamping pressure uniformly without distortion.Clamps 13 rotate to exert even pressure by means of long springs 14connected to angle clamps 13. The free ends of springs 14 fit into slots15 in the opposite angle clamps 13 or clamp supports 18 which arerigidly attached to cooling tray 12 and hinged with hinges 16 to angleclamps 13. Springs 14 provide great mechanical advantage for ease ofoperation and high, exactly reproducible clamping pressure, while thegeometry of the rotating angle clamps 13 resists twisting and unevenpressure, unlike conventional screw or lever clamps.

The polyfoam molding block 2 thus securely immobilized, it is necessaryto cool the eutectic alloy as rapidly and efficiently as possible sothat the alloy becomes "self-sealing" and cannot leak out of the lower"closed" end of the cavity 3 in the polyfoam block 2 which rests uponthe cooling surface of tray 12. Accordingly, the surface of the coolingheat-sink tray 12 should be brushed or etched in order to increasesurface area and improve thermal transfer. When the alloy is poured intocavity 3 the alloy begins to solidify instantly and provides its ownseal. Castings, can thereby be produced very rapidly with littledifficulty, and cooling time can be reduced from hours to minutes.

In FIG. 3 the polyfoam block 2 with the metal casting 9 in place isshown ready for attachment to the treatment tray 8 by means offasteners, shown as thumb screws 7', which pass through holes 6' intreatment tray 8.

FIG. 4 shows the polyfoam block 2 in place on the treatment tray 8,fastened by thumb screws 7' through holes 6' in the treatment tray. Theholes 6' in the treatment tray 8 are situated identically to the holes 6in the cutter tray shown in FIG. 1. The cut cavity is shown already castwith the eutectic alloy casting 9 which has been attached to thetreatment tray 8 by means of the screws 10.

FIG. 5 shows the treatment tray 8 with the polyfoam block removed andthe casting 9 left in correct location by means of screws 10. The nowunused indexing holes 6' are shown for clarity.

Of course, many such mechanical indexing arrangements are possible, butthe essential feature is that the polyfoam block must index to both thecutter tray and the treatment tray in such manner that the cast fieldshaping block will occupy the same location on both trays. In theillustrated example, this was done with an identical pattern of holesand screws in both the cutter and treatment trays; the pattern is thuslocated and the field shaping blocks oriented with perfect accuracy. Inthis manner the relationship between the hot wire pivot position(phantom radiation source), the cut polyfoam block on the cutting trayand the patient x-ray film, in a hot wire cutting system, is identicalto the relationship between the radiation source, the cast metal fieldshaping block on the treatment tray and the patient during treatmentusing a radiation therapy treatment machine.

In use, the treatment machine operator slides the cast metal blocksecured to the treatment tray into the treatment machine and the metalblock then needs no further manipulation to ensure proper orientation orplacement for treatment because the polyfoam block into which the metalblock was cast was identically indexed to both the cutting and treatmenttrays.

For some procedures it is necessary to use a cast block of paraffin orother material rather than the cast eutectic alloy (or other cast metal)block. The apparatus and method described above are suitable for castingand use of paraffin blocks as well as for metal blocks.

Variations and modifications of this invention can be effected withinthe spirit and scope of the invention as described above and as definedin the appended claims.

I claim:
 1. A cooling tray for supporting a generally flat polyfoamblock having laterally spaced parallel opposite sides and generally flatupper and lower surfaces and having a cavity extending therethrough froma top surface to a bottom surface thereof for accommodating afield-shaping molten metal block cast therein,said cooling traycomprising flat heat sink base means for supporting the block, a pair ofupstanding flat parallel wall means affixed to said base means forrespectively engaging the opposite sides of the block along the lengthsthereof, while the block is supported by the base means, a pair ofclamping members each being substantially L-shape and comprised of apair of flanges disposed at right angles to one another, hinge meanspivotally connecting one flange of each of said clamping members to arespective one of said upstanding wall means, said clamping membersbeing pivotal between a first position in which the other flange of eachclamping member engages against the top surface of the block adjacentone side along the length thereof and a second position in which saidother flange of each clamping member stands clear of the block, andspring means for biasing said flanges to swing toward the first positionso that the block is forced downward against the heat sink base means.